A sheet wrapping method, a filament winding method and the like are usually known as a process of the production of fiber reinforced plastic (hereinafter referred to as FRP) tubing such as a golf club shaft and a fishing rod.
In the sheet wrapping method using FRP intermediate materials among these methods, plural FRP intermediate materials are prepared in advance and are then wound around a cored bar one by one, followed by heating to cure the intermediate materials to produce FRP tubing.
Among these plural FRP intermediate materials used to form the FRP tubing, it is preferable to prepare at least one set of FRP intermediate materials having fibers oriented at reverse angles ranging from ±15° to ±75° with respect to an axis of the cored bar. However, when the set of FRP intermediate materials is wound around the cored bar, it is preferable to integrate these FRP intermediate materials in advance to simplify the winding work. Consequently, it is usual to laminate the FRP intermediate material layers on each other to integrate these intermediate material layers prior to the winding work.
A general method of laminating these FRP intermediate material layers on each other is a heat press method. This is a method in which a set of FRP intermediate material layers is overlapped on each other, then put in a heat press and pressed under heating to thereby soften a resin material constituting the FRP intermediate materials, thereby fusing the interface between the FRP intermediate material layers, followed by removing air cells remaining on the fused interface by pressure.
Here, it is important to prevent air cells from being left at the interface between the laminated FRP intermediate material layers with the view of suppressing fine voids generated at a laminate interface of a molded FRP golf club shaft. This is the reason why the optimum condition is set by controlling the temperature, pressure and pressing time of the heat press.
However, in the lamination of the FRP intermediate material layers by using the conventional heat press method, the fusion of the interface and the removal of the air cells can be attained with high accuracy if the content of a resin in the FRP intermediate material layers to be used is 33 mass % or more. However, in the case of using the FRP intermediate material layers having a resin content of 33 mass % or less and particularly less than 25 mass %, the fusion at the interface is insufficient. Therefore, when the integrated FRP intermediate material layers are wound around the cored bar, such problems are caused that the FRP intermediate material layers are uncoupled or that even if the interface is fused, huge voids are generated at the laminate interface of the molded FRP golf club shaft because many air cells are left at the interface.
In addition, when the heat press operation is carried out under the condition of higher temperature and higher pressure than conventional operations in order to improve the above problems, a resin material constituting the FRP intermediate materials is fluidized, giving rise to problems concerning snaking of carbon fibers and a spill of the resin material.
Here, there is a fear that the generation of many voids at the laminate interface of molded FRP tubing and the snaking of carbon fibers inside of the FRP intermediate materials will cause deterioration in mechanical characteristics, such as torsion strength and bending strength, of the FRP tubing. There is also a fear that the spill of a resin from the FRP intermediate materials promotes a variation in the weight of the molded FRP tubing.
As one of methods used to solve the above problems, a method is considered in which a fastening condition of a lapping tape, for example, fastening tension of a tape is controlled or a taping material is selected. It has been, however, confirmed that the effect of the fastening using the lapping tape is confined to the removal of air cells remaining between layers (hereinafter referred to as a straight layer), where reinforcing fibers are oriented in a direction along an axis of the FRP golf club shaft, in the vicinity of the outermost layer and the fastening using the lapping tape has a small effect on the removal of air cells remaining between innermost layers (hereinafter referred to as an angle layer) where reinforcing fibers are oriented at an angle of ±15° to ±75° with respect to the axis of the FRP golf club shaft.